test: fix timing sensitivity in debugger test

test-debugger-util-regression.js was sensitive to timing, which seems
to have changed enough with V8 5.7 to cause this test to fail. Fix the
test to ensure we take debugger steps only at stable states instead of
erroneously taking a step on a partially complete buffer.

PR-URL: https://github.com/nodejs/node/pull/11008


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>